A large number of photosensitive materials are known which have a photosensitive layer on a support and form image by imaging exposure. An example of a system that enables environmental conservation or simplification of image formation includes a technique of forming an image by heat development.
In recent years, reduction of amount of waste processing solutions is strongly desired in the field of photomechanical processes from the standpoints of environmental protection and space savings. Techniques relating to photosensitive heat-developable materials for use in photomechanical processes are required which enables efficient exposure by a laser scanner or a laser image setter and formation of a clear black image having high resolution and sharpness. The heat-developable photosensitive materials can provide users with a simple and non-polluting heat development processing system that eliminates the use of solution-type processing chemicals.
Methods for forming an image by heat development are described in, for example, U.S. Pat. Nos. 3,152,904, 3,457,075 and D. Klosterboer, Imaging Processes and Materials, "Thermally Processed Silver Systems", 8th ed., Chapter 9, page 279, compiled by J. Sturge, V. Walworth and A. Shepp, Neblette (1989). The photosensitive material contains a reducible light-insensitive silver source (e.g., organic silver salt), a photocatalyst (e.g., silver halide) in a catalytically active amount, and a reducing agent for silver, which are usually dispersed in an organic binder matrix. This photosensitive material is stable at an ambient temperature, but when the material is heated at a high temperature (e.g., 80.degree. C. or higher) after light exposure, silver is produced through an oxidation-reduction reaction between the reducible silver source (which functions as an oxidizing agent) and the reducing agent. The oxidation-reduction reaction is accelerated by catalytic action of a latent image generated upon exposure. The silver produced by the reaction of the reducible silver salt in the exposure region provides a black image and this presents a contrast to the non-exposure region to form an image.
Since such image formation methods require no processing solutions such as developers, and provides images only by heating, i.e., without generating sulfur dioxide gas or ammonia gas. Therefore, those materials have become to attract much attention as recording materials that can be used with image-drawing apparatuses utilizing laser light and so forth. Laser image-drawing apparatuses are used in many fields. For example, they are used in image-forming apparatuses for medical use, image-forming apparatuses for photomechanical processes, image-drawing apparatuses for industrial use and so forth.
Those heat-developable recording materials are usually require heating time of 10 to 60 seconds at a temperature of 100.degree. C. or higher.
Various heat development apparatuses have been proposed. For example, there have been known a method utilizing contact of a photosensitive material with a heat plate or a heat roller to attain heating by heat conduction, a method utilizing radiation heating by passing a photosensitive material through an oven, a method utilizing heat generation inside a layer of photosensitive material caused by electromagnetic wave, a method utilizing heat generation of resistive materials (carbon black etc.) upon applying an electric current, and so forth. Whichever method is employed, it is very important to maintain the whole surface of photosensitive material at the same temperature, and heat development apparatuses are variously devised for this purpose. However, it is actually impossible to keep the material within a temperature distribution within a temperature difference of less than .+-.0.5.degree. C. The development may be performed for a wide area, for example, A1 or B1 size, for photosensitive materials used for mechanical processes. In such a case, in particular, it becomes still difficult to maintain a uniform temperature distribution. Therefore, it is desired for photosensitive materials to be used to have large latitude for the fluctuation of heat development temperature.
In recent years, as a heat-developable recording material for mechanical processes, materials utilizing transmission phenomenon by an ultrahigh contrast agent are under development. However, it has been found that such materials have a problem that fluctuation of development temperature causes extremely large fluctuation of the degree of ultrahigh contrast. If the degree of ultrahigh contrast is fluctuated, the size of half tone dot area or a line thickness will be fluctuated. Therefore, basic performance as a heat-developable recording material may not be obtained. In order to enable to use such heat-developable recording materials for that field, it is a very important theme to improve performance fluctuation depending on heat development temperature.
A still more important theme for heat-developable recording materials is improvement of storage stability. Because heat-developable recording materials also contain regents required for image development in a photosensitive layer beforehand, they cause deterioration such as increase of fog, reduction of photographic sensitivity and so forth. Therefore, they suffer from the drawback of extremely short shelf life.
As high contrast agents for forming high contrast images, there have been known the acylhydrazine derivatives disclosed in U.S. Pat. Nos. 5,464,738, 5,512,411, 5,496,695 and 5,536,622, the acrylonitrile derivatives disclosed in U.S. Pat. Nos. 5,545,515 and 5,635,339, the malondialdehydes disclosed in U.S. Pat. No. 5,654,130, the isoxazoles disclosed in U.S. Pat. No. 5,705,324 and so forth. As methods for accelerating the development, there has been disclosed addition of certain materials into image-forming layers together with the ultrahigh contrast agents, and such materials include the amine compounds disclosed in U.S. Pat. No. 5,545,505, the hydroxamic acids disclosed in U.S. Pat. No. 5,545,507, the hydrogen donors disclosed in U.S. Pat. No. 5,637,449 and so forth.